The ability of cells to interact with extracellular matrix preserves organ integrity and function, but may also aberrantly contribute to neoplasia. Such adhesive recognitions are coordinated by integrins, members of a large gene superfamily of cell adhesion receptors. The applicant has identified, cloned, and characterized an alternatively spliced variant of the human betai integrin subunit, designated beta1C (formerly beta1S), containing a unique 48 amino acid cytoplasmic domain. Transient expression of beta1C blocks cell cycle progression and DNA synthesis in genetically engineered cells. Deletion mutant analysis shows that two spatially distinct beta1C cytoplasmic sequences, G1`n795-Gln802 and Trp813-Pro825, play a role in negatively regulating cell growth . Furthermore, in vivo expression of B1C correlates with a non-proliferating cell phenotype and is down-regulated in neoplasia. Therefore, a potential role of beta1C in normal and cancer cell growth is hypothesized, and will constitute the focus of the present proposal. In the first specific aim, beta1C functions will be studied by characterizing beta1C pairing with specific alpha(s), ligand recognition, association with cytoskeletal proteins and activation of tyrosine kinase- dependent signaling pathways. The beta1C and the wild type beta1 subunit functions will be compared in order to identify differential effects due to their cytoplasmic domains. Molecular analysis of the beta1C cytoplasmic domain will be carried out in the second specific aim. Here, targeted site-directed mutagenesis and sequence-specific antibodies will antibodies will elucidate the reciprocal contribution of beta1C sequences G1n795- Gln802 and Trp813-Pro825 in the regulation of cellular growth. To address the potential contribution of beta1C to malignant cell transformation, these experiments will be carried out in tumorigenic versus non-tumorigenic epithelial prostate cells, previously characterized for their beta1C expression. Finally, the third specific aim will characterize the function of beta1C in vascular endothelium, with respect to its participation in endothelial cell proliferation, migration, and angiogenesis, a critical step in tumor growth. These experiments will be carried out by genetic manipulation of bovine and human endothelium, and corroborated by beta1C mutagenesis and sequence-specific inhibitory antibodies. Overall, the proposal is designed to elucidate a novel aspect of integrin-dependent control of cellular functions in normal and cancer cells. These studies will, therefore, enhance our understanding of the mechanisms responsible for aberrant cell growth and tumorigenesis.